EVOLUTION AND CONSERVATION BIOLOGY OF THE GALAPAGOS-OPUNTIAS (CACTACEAE)

The Galapagos Islands, located in the eastern Pacific Ocean, are home to 6 species and 14 varieties of Opuntia, all endemic, Animals pollina te these cacti and disperse their seeds. Individual plants are slow-gr owing and appear to be long-lived. Population differentiation and spec iation are associated with major barriers to dispersal or with strong clinal variation in the environment. Both arborescent and shrubby opun tias occur in the Galapagos. Although herbivory by tortoises and iguan as has been suggested as the selection pressure for evolution of tree- sized taxa, competition with other plants or wind-induced mortality of large trees cannot be ruled out. The ancestry of the Galapagos specie s, and their relationship to each other, remain in doubt. Most Galapag os Opuntia taxa are not now in danger of extinction. However, several taxa are vulnerable or threatened, primarily because of introduced fer al herbivores. Although these plants have been studied by naturalists for a century and a half, there are still significant gaps in our know ledge of their distribution, systematics, ecology, and evolution.